Method and apparatus for controlling nucleation in self-assembled films

ABSTRACT

A method of forming a self-assembled film with periodic nanometer dimension features (e.g., holes) on a substrate includes the steps of providing film precursors on the substrate, wherein the film precursors are maintained in an amorphous state. Where the film precursors are block copolymers, a heating member is provided. The substrate and the heating member are then moved relative to one another so as to raise the temperature of a portion of the film precursor on the substrate above its glass transition temperature. Relative movement between the substrate and heating member continues until a self-assembled crystalline film is formed over the surface of the substrate. In an alternative embodiment, a pH dispensing member is provided to dispense a pH adjusting agent onto the substrate that promotes self-assembly of a crystalline film.

FIELD OF THE INVENTION

The field of the invention generally relates to self-assemblednano-structured films. More particularly, the field of the inventionrelates to methods and processes used to control the nucleation in theself-assembly of nano-structured films.

BACKGROUND OF THE INVENTION

The semiconductor industry is increasingly being driven to decrease thesize of semiconductor devices located on integrated circuits. Forexample, miniaturization is needed to accommodate the increasing densityof circuits necessary for today's semiconductor products. Increasedpacking density and device size reduction has forced semiconductordevice structures such as, for instance, transistors to be located evercloser to one another.

Unfortunately, conventional techniques of reducing component size willsoon be reaching their limits. For example, conventionalphotolithography suffers from the diffraction limit and, consequently,cannot define patterns generally smaller than 100 nm. Advancements inlithographic methods such as e-beam lithography have been able createfeatures as small as 10 nm. However, e-beam lithography is a sequentialprocess and suffers from low throughput. Thus, e-beam lithography is notwell suited for commercial applications which necessarily require highthroughput.

As the trend toward miniaturization continues, the size of features insemiconductor-based devices will be on the nanometer scale. Features maybe formed from molecules or even single atoms. The problem is that theprecise location of molecules or atoms on a substrate is difficult tocontrol. Control of the precise location of features on a substrate hasbeen essential, however, in CMOS-based processes. ConventionalCMOS-based process require precise registration of features over anentire substrate. Such substrates can be particularly large, forexample, having diameters as large as 12 inches. Any new process forforming semiconductor-based features needs to preserve this precision onthe wafer scale in order to make the process commercially feasible.

The barrier to nanometer scale lithography has given rise to attempts touse self-assembled molecules as a potential means to form nano-scalestructures on a substrate. These techniques generally involve blockcopolymer or protein crystals with unit cell dimensions being on theorder of nanometers. A subsequent step, which may take the form of anetching step, is used to transfer the formed pattern onto the substrateor overlying layer as in conventional photolithographic processes. Onechallenge, intrinsic to all self-assembly processes, is thepoly-crystalline nature of the resulting film. Because of thesimultaneous or near-simultaneous nucleation of many ordered domains,also known as crystallites over the entire surface of the substrate, thepoly-crystalline morphology cannot be avoided. The maximum grain size ison the order of 1 μm. Consequently, there is no long-range order in theresulting crystalline films as is generally required for CMOS-basedtechnology.

Various attempts have been undertaken to increase the long-range orderof self-assembled nano-structured surfaces. In one method, for example,the substrate on which the film is formed contains regular patterning.Patterning is carried out by topographically or chemically modulating asubstrate using interference lithography. The periodically-modulatedsubstrate then provides for long-range order to the self-organizedsystem. In another method, graphoepitaxy is used to induce orientationand positional ordering of block copolymer through artificial surfacepatterning such as the grooves of a diffraction grating. See e.g., C. A.Ross et al., Nanostructured Surfaces with Long-Range Order forControlled Self-Assembly, NSF Nanoscale Science and Eng. Grantees Conf.,Dec. 16–18, 2003. In the latter process, well-ordered structures canreportedly be formed within the grooves of the grating.

Still others have attempted to use rapid solidification to orient blockcopolymer micro-domains. U.S. Patent Publication No. 2003/0118800discloses such a process that uses rapid solidification from a solventto form patterns of micro-domains in thin films of block copolymer.

There remains a need for a method of controlling the nucleation ofself-assembled thin films. Preferably, the method can be used to createlong-range order over the entire surface of a substrate and not just inone or more micro-domains on the substrate. The method wouldadvantageously have high throughput such that the method could beemployed in commercial applications to create nano-scale structures.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method of forming a self-assembledcrystalline film on a substrate includes the steps of providing filmprecursors on the substrate, wherein the film precursors are maintainedin an amorphous state. A heating member is provided adjacent to thesubstrate and the substrate and heating member are subject to relativemotion between the two. By moving the heating member and/or substraterelative to one another, a portion of the film precursors on thesubstrate is heated above the glass transition temperature. The processcontinues until a 2D self-assembled crystalline film is formed on all orsubstantially all of the surface of the substrate. It is an object ofthe invention to maintain a single ordered domain across the entirewafer surface throughout the self-assembly process.

In one aspect of the invention, the heating member is stationary whilethe substrate is moved with respect to the stationary heating member. Inan alternative aspect of the invention, the substrate is stationarywhile the heating member is moved with respect to the stationarysubstrate. In yet another alternative, the substrate and the heatingmember are both moving.

The method described above may be followed by one or more steps ofselectively removing one species of the self-assembled crystalline filmformed on the substrate.

In still another aspect of the invention, a method of forming aself-assembled crystalline film on a substrate includes the steps ofproviding a film precursor on a substrate. The film precursor may bemaintained in an amorphous state. A dispensing member is provided abovethe substrate and contains a pH adjusting medium. The pH adjustingmedium is dispensed onto the substrate. The pH adjusting medium promotescrystallization of the film precursor into a self-assembled film.

The method described above may be implemented with a film precursor inthe form of a polymer or protein. In addition, the pH adjusting mediummay contain a viscosity altering agent for the purpose of controllingthe speed of spreading of the film precursor.

In still another aspect of the invention, a device for forming aself-assembled film on a substrate containing a film precursor includesfirst and second heating members. The second heating member is disposedadjacent to the first heating member. The second heating member isseparated from the first heating member by a gap, which in one preferredaspect of the invention, is a v-shaped gap. A moveable pusher member isprovided for advancing the substrate from the first heating member tothe second heating member.

In one aspect of the device, the first heating member has a temperatureabove the order-disorder transition temperature (T_(OD)) of the filmprecursor while the second heating member has a temperature above theglass transition temperature (T_(glass)) of the film precursor but belowT_(OD).

In another aspect of the device, the first heating member has atemperature below the glass transition temperature (T_(glass)) of thefilm precursor while the second heating member has a temperature abovethe glass transition temperature (T_(glass)) of the film precursor butbelow T_(OD).

It is an object of the invention to provide a method of formingnano-scale structures on a substrate. It is a further object of theinvention to provide a method of forming self-assembled crystalline filmon a substrate having long-range order across all or substantially allof the surface of the substrate. Advantageously, the methods may be usedin commercial applications because of their high throughputcapabilities. Additional objects of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a substrate such as, for example, a siliconsubstrate having a film (e.g., silicon dioxide) disposed on an uppersurface thereof.

FIG. 1B illustrates the substrate of FIG. 1A having film precursors suchas, for example, a di-block copolymer film disposed thereon.

FIG. 1C illustrates the substrate of FIG. 1B with a moving heatingmember located above the upper surface of the substrate containing thefilm precursors. FIG. 1C also illustrates the advancing front ofself-assembled crystalline film.

FIG. 1D illustrates a substrate having a self-assembled di-blockcopolymer film consisting of a single grain pattern of holes. Thediameter of the holes and the period of the pattern (crystal) aredetermined by the lengths of the two polymer blocks of the copolymer.The length is typically on the order of about 10 nm.

FIG. 2 illustrates a top down view of a substrate (with film precursors)showing the advancement of the self-assembled 2D crystalline region.

FIG. 3 illustrates a substrate being disposed on a thermal backingmaterial such as, for example, a thermal reservoir or heat sink.

FIG. 4 illustrates an alternative aspect of the invention wherein adispensing member containing a pH adjusting medium is used to promotecrystallization of a biological (e.g., protein) film precursor into aself-assembled film.

FIG. 5 illustrates two thermally conductive blocks or members used in analternative self-assembly process

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A illustrates a substrate 2 such as, for example, a siliconsubstrate 2 having an intermediate film 4 (e.g., silicon dioxide)disposed on an upper surface thereof. The substrate 2 may take the formof a wafer or the like which are commonly used to produce semiconductordevices. The substrate 2 may be formed, for example, from silicon,gallium arsenide (GaAs), gallium nitride (GaN), or the like. Thecomposition of the intermediate film 4 may be selected, for example,based on its thermal conductivity level. Illustrate examples ofintermediate films 4 include silicon dioxide (SiO₂), SiN_(x), amorphoussilicon, and metals such as aluminum and tungsten.

Referring now to FIG. 1B, film precursors 6 are provided on the uppersurface of the substrate 2. In one aspect of the method, the filmprecursors 6 include block copolymers. For example, the film precursors6 may include a polyisoprene (PI)—polystyrene (PS) di-block copolymer(PI-PS). Another example of a block copolymer system includespoly(styrene-block-dimethylsiloxane) (P(S-b-DMS)). In this system, PDMShas a strong resistance to many types of reactive ion etch processeswhile polystyrene (PS) generally does not.

Still other block copolymer systems may be used in accordance with themethods contemplated herein. For example, systems where one component isdegraded preferentially in response to radiation may be employed (e.g.,polymethylmethacrylate (PMMA) is known to be degraded on exposure to anelectron beam or ultraviolet light while polystyrene (PS) is morestable). Block copolymer systems can also be used where one component issusceptible to chemical treatments to alter etching rates. For instance,in a PS system incorporating polyisoprene (PI) or polybutadiene (PB), PSmay be etched at a higher rate than either PI or PB when the system isexposed to osmium tetroxide (OsO₄) and later subject to CF₄/O₂ reactiveion etching. It should be understood that the methods described hereinmay be used with any number of block copolymer systems known to thoseskilled in the art. For example, block copolymers can be made withdifferent lengths of individual polymers leading to different clustersizes.

Referring back to FIG. 1B, the film precursors 6 may be disposed ontothe substrate 2 by spin coating the film precursors 6 onto a substrate 2containing the intermediate film 4. Alternatively, the film precursors 6may be deposited on the substrate 2 by evaporation. For example, in oneaspect of the method, a PI-PS di-block copolymer system may be spun on asubstrate 2 covered with silicon dioxide. The film precursors 6 aremaintained on the substrate 2 in an amorphous state (i.e., no formationof crystallites of any significant size). Generally, very smallcrystallites can easily reorient themselves to incorporate into a largecrystallite nearby whereas large crystallites cannot because of theprohibitively high energy threshold. Thermal processes typically cannotovercome the high energy threshold levels required for largecrystallites.

Referring now to FIG. 1C, a heating member 8 is provided to heat aportion of the film precursors 6 above their glass transitiontemperature (T_(glass)). It is known that at temperatures aboveT_(glass), the block copolymer phase segregates and the self assembledfilm 10 is composed of clusters of one of the polymers (e.g., PI spheresor balls) regularly distributed throughout a polymer matrix (e.g., PSmatrix). See, e.g., C. Harrison et al., Lithography With A Mask of BlockCopolymer Microstructures, J. Vac. Sci. Technol., B16, p. 544 (1998).The above publication is incorporated by reference as if set forth fullyherein.

As illustrated by a PI-PS di-block copolymer system, self-assembly intoa 2D film 10 is driven by the interaction between the PI spheres orballs. If at any moment there is only a single crystal on the substrate2 surface, film near the perimeter of the crystal (or nucleation site)will self-assemble into part of the growing 2D crystal film 10. If,however, the entire substrate 2 surface were heated at once, manycrystallites form and grow simultaneously at different locations on thesubstrate 2. Unfortunately, the growth fronts from the variousnucleation sites meet and result in an orientational mismatch thatcannot be reconciled because it requires reorientation of the entirecrystallite (which has a μm size). Consequently, there is no long-rangeorder of resultant film. The present method overcomes this problem bypropagating a moving front of a single domain of the self-assembled film10—thereby avoiding simultaneous nucleation across the substrate 2surface.

The heating member 8 may include an internal heating element (not shown)or be heated by some external source (e.g., heated by radiation or thelike). In one aspect, the heating member 8 is in the form of a heatedtip such as that shown in FIG. 1C. The heated tip generally includes anangled or triangular portion that terminates in pointed tip.

Relative movement is then initiated between the heating member 8 and thesubstrate 2. In a preferred embodiment of the method, the relativemovement starts at one side or edge of the substrate 2 and progresses toan opposing side or edge. In this regard, the nucleation of a singletwo-dimensional (2D) self-assembled crystalline film 10 is controlled bythe relative movement between the heating member 8 and substrate 2. In apreferred aspect of the invention, a moving front or region progressesacross the surface of the substrate 2. In essence, nucleation of asingle 2D crystalline film 10 is initiated at one point on the surfaceof the substrate 2 and is spread throughout the entire surface of thesubstrate 2. Preferably, the 2D crystalline film 10 may be formed in asingle pass of the heating member 8 and/or substrate 2.

The heating member 8 is heated such that the film precursors 6 locatedadjacent to the heating member 8 are able to be heated to a temperaturethat exceeds T_(glass). In this regard, only a portion of the filmprecursors 6 located adjacent to the heating member 8 are heated aboveT_(glass).

Still referring to FIG. 1C, relative movement between the heating member8 and the substrate 2 causes a front of self-assembled film 10 to movein the direction of arrow A. The front propagates across all orsubstantially all of the surface of the substrate 2. In one aspect ofthe invention, the heating member 8 is stationary while the substrate 2is moved. In an alternative aspect of the method, the substrate 2 isstationary while the heating member 8 is moved.

With reference now to FIG. 2, in a preferred aspect of the method, heatflow within the vicinity of the advancing front of self-assembled film10 is limited to a value such that the width of the area just in frontof the advancing front (represented by arrows B in FIG. 2) is less thanor narrower than the range of interaction between adjacent PI balls orspheres. Generally, the method works best with a large or steeptemperature gradient along the surface of the substrate 2. If the widthof the elevated temperature field is too large, nucleation of the film10 may result too far beyond the advancing front—thereby causingmultiple polycrystalline micro-domains.

In one aspect of the invention, the width and steepness of thetemperature gradient created on the surface of the substrate 2 may becontrolled by the heat transfer properties of the substrate 2. Forexample, the composition of the intermediate film 4 may be chosen toincrease the gradient created on the layer of film precursors 6.Alternatively, as shown in FIG. 3, the backside of the substrate 2 maybe placed in thermal contact with a thermal member 20 which may include,for example, a constant temperature thermal reservoir or heat sink.

An important requirement for the annealing set up is the ability ofsustaining a steep temperature gradient. An alternative annealing set upis shown in FIG. 5, in which the precursor coated substrate 2 is incontact with and is moved across a gap 40 formed between two blocks 42,44 of a thermally conductive material, for example, aluminum. Thetemperature of the two blocks 42, 44 can be maintained at, for example,above and below the T_(glass) of the film precursor 6. Alternatively,the temperature of the two blocks 42, 44 can be maintained to be aboveand below T_(OD), the order-disorder transition temperature. T_(OD) isthe temperature above which the entropy term dominates Gibb's freeenergy and the disordered state, as opposed to the ordered state, isfavored thermodynamically.

As seen in FIG. 5, in one aspect of the process, the precursor coatedsubstrate 2 is placed on a first heated block 42. The substrate 2 isthen pushed in the direction of arrow A via a pusher member 46 acrossthe v-shaped gap 40. The gap 40 between the first and second heatedblocks 42, 44 is on the order of approximately 1 μm. In one aspect ofthe process, the first heated block 42 is maintained at a temperatureabove T_(OD) while the second heated block 44 is maintained at atemperature above T_(glass) but below T_(OD). The two heated blocks 42,44 may be maintained at different temperatures to maintain a temperaturedifference of about 50° C. between the respective upper contact surfacesof the heated blocks 42, 44. Self assembly takes place over the secondheated block 44 and completes when the substrate 2 is completely pushedonto the second heated block 44.

The speed of relative movement between the substrate 2 and the heatingmember 8 is preferably controlled to fall below a threshold value whichcorrelates to the speed of self-assembly of the 2D film 10. The relativespeed should thus be slow enough for self-assembly to take place. At toohigh a rate, self-assembly of the film precursors 6 will not take place.

After forming the 2D self-assembled film 10 on all or substantially allof the surface of the substrate 2, the pattern may then be transferredto the intermediate film 4 and/or substrate 2. For example, theintermediate film 4 may be patterned using reactive ion etchingtechniques followed by subsequent removal of the remaining polymer film10.

As one illustrative example, in a PI-PS di-block copolymer system, thesubstrate 2 containing the 2D self-assembled film 10 can be subject to avacuum annealing process which forms spherical domains (PI spheres) inthe self-assembled film 10. The PI spheres can then be selectivelyremoved by ozone treatment. The voids may then be exposed by subjectingthe substrate 2 to CF₄ reactive ion etching or CF₄/O₂ reactive ionetching. U.S. Pat. No. 5,948,470, which is incorporated by reference asif set forth fully herein, describes such a process to selectivelydegrade and remove components of block copolymers. In addition, a wetchemical pattern transfer process (or other process known to thoseskilled in the art) may be used to transfer the pattern into thesubstrate 2.

FIG. 1D illustrates a substrate 2 having a silicon dioxide mask 12 witha single grain pattern of holes 14. The pattern of holes 14 may then beused to grow a variety of devices or components that are registered withthe substrate 2 with nanometer precision. The holes 14 provide aframework or foundation on which subsequent processes can be based toform nano-scale structures.

FIG. 4 illustrates an alternative process used to form 2D self-assembledfilms 10. In FIG. 4, the substrate 2 is covered with a film precursor 30which may take the form of a biological-based precursor such as aprotein. In one aspect of the invention, the protein (or otherprecursor) self-assembles into a film 10 upon exposure to an appropriatechemical environment. For example, the protein may crystallize above orbelow a particular pH. The film precursor 30 is preferably maintained onthe substrate 2 in an amorphous state as described above. A dispensingmember 32 is provided above the surface of the substrate 2. Thedispensing member 32 may take the form of a dropper, pipette, ormicro-pipette for example. The dispensing member 32 includes therein oris otherwise connected to source of a pH adjusting medium 34. The pHadjusting medium 34 may comprise an acid, base, or even bufferedsolution. The pH adjusting medium 34 is chosen to promote theself-assembly or crystallization of the film precursor 30 into aself-assembled film 10.

As seen in FIG. 4, the pH adjusting medium 34 is dispensed from thedispensing member 32 onto the surface of the substrate 2. Preferably,the pH adjusting medium 34 is released in a controlled rate such that anadvancing front of the pH adjusting medium 34 causes the self-assemblyof the film precursor 30. Preferably, the speed of the advancement ofthe perimeter of the deposited pH adjusting medium 34 (represented byarrows C in FIG. 4) is low enough to permit the ordered crystallizationof the film precursor 30. In one aspect of the process, a viscosityaltering agent is added to the pH adjusting medium 34. The viscosityaltering agent may include a viscosity enhancing agent (to increaseviscosity) or a viscosity reducing agent (to decrease viscosity). Theviscosity altering agent preferably slows or accelerates the speed atwhich the front of the pH adjusting medium 34 travels across the surfaceof the substrate 2. Alternatively, the wetting property of the filmprecursor 30 may be altered to adjust the speed at which the pHadjusting medium 34 flows.

In a preferred aspect of the invention, the pH adjusting medium 34 isdispensed in a central region of the substrate 2, as is shown in FIG. 4.Preferably, the pH adjusting medium 34 naturally flows outwardly towardthe edges of the substrate 2. In an alternative embodiment of theinvention, the substrate 2 may be rotated to assist the movement of thepH adjusting medium 34 across the surface of the substrate 2.

While embodiments of the present invention have been shown anddescribed, various modifications may be made without departing from thescope of the present invention. The invention, therefore, should not belimited, except to the following claims, and their equivalents.

1. A method of forming a self-assembled block copolymer film on asubstrate comprising: providing first and second heating membersseparated by a v-shaped gap, wherein the first heating member ismaintained at a temperature above T_(OD) and the second heating memberis maintained at a temperature below T_(OD) but above T_(glass);providing block copolymer film precursor on the substrate in anamorphous state; placing the substrate in contact with the first heatingmember; moving the substrate across the v-shaped gap to the secondheating member to form a self-assembled block copolymer crystallinefilm.
 2. The method of claim 1, wherein the width of the v-shaped gap isapproximately 1 μm.
 3. The method of claim 1, wherein an intermediatefilm is disposed between the substrate and the film precursor.
 4. Themethod of claim 1, further comprising selectively removing one or moreregions of the self-assembled film formed on the substrate.
 5. Themethod of claim 4, further comprising the step of removing the remainderof the self-assembled film formed on the substrate.
 6. The method ofclaim 1, wherein the first and second heating members are maintained ata temperature differential of about 50° C.
 7. The method of claim 1,wherein the substrate comprises a wafer.
 8. The method of claim 1,wherein the substrate is moved across the v-shaped gap by a pushermember.